Method of making electrical strain gauges



May 28, 1946.

B. J. CAMPBELL ETAL 2,401,049

METHOD OF MAKING ELECTRICAL STRAIN GAUGES Filed Dec. 13, 1943 2 Sheets-Sheet 1 hoe/22577 y 1946; 5. J. CAMPBELL ETAL 2,401,049

METHOD OF MAKING ELECTRICAL STRAIN GAUGES Filed Dec. 13, 1943 2 Sheets-Sheet 2 composed of two cover iig.

Patented May 28, 1946 I UNlTED STATES PATENT OFFICE iun'rnon or Maxiiz n ilc'rnicm. STRAIN I Bernard J, Campbell an d Marietta Hilton, San

Diego, Calif asslgnors to'Consolidated Vultee rcraft Corporation, poration of Delaware San Diego, Calif., a cor- Application December-13,1943, Serial No. 514,118

' 7 Claims. (01140-71) The invention relates to the fabrication of electrical strain gauges.

Strain gauges. for-measuring minute deflections 01' the are usually in. the form of thin flexible patches strips or sheets of thin paper, such as onion skin, cemented together with a resistance element formed of a predetermined length of resistance wire interlaid between the sheets, the ends of which are joined to the lead-conductors for connection to an 8160-. used, these gauges to a surface on the structure to be tested with which they become integral and the resistance elementsmust be extached. An imperceptible variation in' the length use. i

Fig. 8 is a perspective illustrating the lower Y cover-strip on the bed and the resistance element surfaces to which they are attached wound to form the resistance element and secu'red to the ends of the lead-conductors.

Fig, 9 is a similar view after the top cover-strip has been cemented to the underlying strip and over the resistance element and the lead-conductors.

Fig. 10 is a perspective on an enlarged scale illustrating the connections between the ends of the resistance element and the lead conductors.

Fig. 11 is a section 1 after the jig hasbeen assembled-for clamping One object of the invention is to provide an 1 improved method for expediting the manufacture of electrical strain gauges and by which accuracy in resistance value 01' the resistance element can be readily and accurately controlled.

Other objects of the invention will appear from the detailed description,

The invention consists in the several novel features which are hereinafter set forth and more particularly defined by claims at the conclusion hereof.

In the drawings: 3

Fig. 1 is a plan of an apparatus embodying the in ention and which is used forcarrying'out the improved method.

I 4 Fig. 2 is a perspective. parts being broken away.

Fig. 3 is a perspective of the jig with an assembled strain gauge' clamped therein.

Fig. 4 is a perspective of the bed-block of the Fig. 5 is a perspective of the plate or sheet of Jig. 4

Fig. 6 is a perspective of the top or clamping section ofthe jig.

Fig. 7 is a perspective of the resilient mat which is placed over the cover-strip and fits under the top section of the jig.

insulation which fits over the bed-block oi the a the gauge while the The unbared portion of cement is cured or dried. Fig. 12 is a section cover sheets 0, c which are cemented together with the endsoi' the resistance element secured by soldering at b to the bared ends a of the leadconductors a, a, respectively, which are provided with hooks a which the ends of the looped.

The invention is exemplifled'in apparatus which comprises a base IS on which the operative parts are supported. A bracket i8 is mounted on one end of the base ii for supporting a reel or spool I! of resistance wire l8 which is drawn from the spool in successively making the strain gauges. A base-member tion of the base l5. Binding posts 20, to which the lead-conductors a. a for the strain gauges are adapted to be connected, are secured in the base and are connected by conductors 2i to a galvanometer (not shown) of usual construction,

or other instrument for measuring and indicating the resistance value in the finished strain gauge.

ductors adjacent its hook. a is detachably gripped in a clamp 22. Each clamp 22 is-mounted on a bar 23 which is longitudinally slidable for shifting the clamp mounted thereon independently and adjusting the hooked end a of one leadconductoraround which wire 18 is looped. Bars 23 are confined for longitudinal sliding movement on the base-member l9 between guide bars 24 and cross-bars 25 which are fixed to the bars 24. Each of the adjustingbars 23 is adjustable longitudinally by a "screw 28 which is screw threaded to an end flange 21 between the'bars 24. The inner end of each screw 26 is connected taken on line ll-ll of Fig.

taken on line i2.l 2 of Fig; 1. v The invention provides for fabricating a strain 1 or suitable means through element are adapted to be i9 is fixed on the other end poreach of the lead-com exemplifies a construction in which the oi the wire forming value desired in the gauge.

to longitudinally adjust said bar by a screw 29 and a nut 30. The outer end of each screw 18 provided with a thumb wheel 3! whereby the .screw may be rotated toimpart sliding movement to the bar 23 to which it is connected. Each clamp 22is connected to one of the adjustable bars 23 by a screw 33 and a rubber washer 34 around the screw which presses the clamp downwardly to detachably grip a leadconductor and secure it on bar 23. The clamps 22 can be raised to facilitate removal of the lead-conductor. The inner end a of each lead-conductor is bared of insulation and provided with a hook a" for conductive connection to the resistance element. The other end of each lead-conductor is also bared for conduc tive connection to the binding posts 213. This hook ends a of the lead-conductors which are gripped by the clamps'22 may be independently adjusted longitudinally, and are adapted to receive loops the resistance element.

A jig is provided for clamping the assembled gauge for drying the cement or adhesive'used to bondthe cover sheets together. This jig comprises a bed on which the cover strip is laid.

. This bed comprises a rectangular block as of electrically non-conductive material and a plate or sheet 36' of Micarta or other heat resisting and electrically non-conductive material which fits on the top of the .block 36. A series of vertical pins 40 have their lower ends fixed in the block 38,.

extend through holes in the plate 38' and through thecover-strip and proiect above the bed and constitute means around which the wire may be wound or looped to form the resistance element b of the desired length or with the resistance The bed composed of block 36 and plate 36 is adapted to be removably' secured on the base between an abutment 31 anda thumb screw 88 which is screw threaded I to .a block 39. The cover-strip c is laid on the bed and its ends are downturned and confined between the sides of the block 88, the abutment 31 and the screw 38, and the pins 48 pass through the cover-stripe so that the cover-strip c will be firmly and flatly held on the bed.

the insertion and accuses the series of forming pins t0, thence around the hook a on the lead-conductor a and the free end of the wire is then hooked to a resilient tension device. This device consists of a vertically extending coil spring 52, the lower end of which is fixedly secured in the base it and the upper end of which is free to flex transversely. By hooking the free end of wire it to a coil of the spring 52 in greater or less distance from its 'iixed lower end, its lever action may be decreased or increased tovary the tension on the wire, according to that desired for wire is of different diameters. This spring 52 is closely wound so that fine wire will be frictionally gripped or wedged between its coils. The diameter of this wire may be .001" in thickness and this spring provides a quick and easy method of securing the end of the wire and for varying the force of the spring for proper degree of tension on wire of various sizes and types, by selective connection of the wire to different coils which increases or decreases the lever action of the spring. action of the spring, a more rugged spring, which is less likely to be damaged, can-be used to provide the very sensitive tension required. The drag on the spool l1 and the resiliency oi hook 1 Kill are suflicient to keep the reach of wire between h cover-strip c jig under the desired tension been wound around the formfree end has been attached the spool and the after the wire has ing pins 40 and its to the spring 52.

The method of making strain gauges and the operation of the apparatus are as follows: A will be placed on the bed composed of assembled plate $8 and block 3t and pressed bed will then be clamped between abutment 31 on Y sulated portion of each lead conductor will then The jig comprises an upper block 42 of elecv trically non-conductive material for applying clamping pressure to-the assembled strain gauge on the bed. An elastic mat d3 of non-conducting soft rubber-fits under the block 82 for engaging the to cover-stripe. Said mat is provided with holes through whichthe forming pins so and the-guide posts ll are adapted to pass. Block 62 is provided with holes for posts M which guide said blocks ,rectllinearly to The mat '43 provides a resilient clamping surface, and distributes the pressure from block is over the uneven surface resulting from the interlaying ofthe resistance wire between the strips c and c.

with hooks for engaging a A clamp 45 is provided studs 68 on the sides of block 35 straddles the block 42 and mat 43, and is provided with a screw 61 for clamping .the assembled strain gaugein the Jig.

The wire i8 from spool i1 is led through andis guided by a hook 49 on a resilient arm bfl'which ismounted at the upperend of a post 5 I, the lower end of which is secured in the base H. In windand from the bed.

the lead-conductor a.

. value of the element ing or looping the wire-l8 to form the resistance element b, the wire is drawn from the spool',' ll through hook l9, looped around the lead-conductor a, thence successively around the book a". on

' to be shortened while the surplus be placed into and gripped by one of the clamps 22 with its bared end a and hook a" overlying one end portion of the cover sheet 0. as illustrated in Fig. 2. The wire 68 from guide hook 69 will be looped around the hook a" of the lead.- conductors a and thence successively around the forming pins 60,. thence around the book a on and its free end will be hooked around the spring 52 and between the coils thereof, so the spring will exert sumcient tension on the wire to hold it taut without stretching the wire. The galvanometer con.- nected to the binding posts 22 will then be By utilizing the lever switched to measure and indicate the resistance b which has been formed between the hooks a while the wire remains under tension to cause it to retain its cntour. Ifthe resistance measurement is plus, the operator will adjust screws 8| to move one or both of the bars 24 inwardly and cause the length of wire between boom a of wire is taken up by spring-52 which keeps the wire until the indicated resistance value on the galvanometer is the same as that desired in the strain gauge. 1! the] indicated measurement resistance value is minus, either or both of the screws 3| will be adjusted to increase the length or the wire becorrect rce is tween hookso until the of the lead-conductors 4 desired predetermined resistance value.

a between the guide posts 4].

permanent bond from the jig by loosening ment, a strandof indicated. The spring 52 will then be flexed for thisincrease. The element b will then have the the operator will cover strip and with athin film coat the exposed surface the. resistahce element thereon,

of liquid adhesive such as Duco household cement or Radio service cement w hile the bed of the jig remains in fixed position on the base. Next, the operator will fixedly join the hooks a of the lead conductors to the resistance wire at the ends of the element by drops of solder, as indicated at M. The free end of the resistance wire will be severed at the outer side of one of the hooks a and the supply end of the wire will be severed at the outer s'ideof the other hook a and both will be removed from the jig. Next, the cover strip 0' will be pressed over pins 40 and against the adhesive on the strip 0 and Next, the rubber mat 43 will be placed on the top cover sheet 0' betw' v the cover sheets and the the cement is 'being ured to set and form a resistance element. Tang can be placed in an The completed strain gauge is then stripped off the forming pins 40 and will be in readiness for use. While one strain gauge is being dried and cured,

can be provided,

gauges can proceed continuously.

Having thus described the invention, what we claim as new Patent is: v

1. That improvement in *making electrical and .desire to secure by Letters Next,

strain gauges having cover-strips with an inter- I laid resistance element and'with lead-conducthe ends of the elementwhich comprises, winding, to form the. resistance eleare held under a 75 prises, winding, to form the resistance element, a strand of wire over a cover-strip and looping it around the ends of a pair of lead-conductors, retaining the loops of wire under yielding tension, varying, by adjustment of-the conductors,

sion in their adjusted relation.

4. That improvement in making electrical strain gauges having cover-strips with an interlaid resistance element and with lead-conductors attached to the ends of the element whichcomto for-m the resistance element, cover-strips pair of leadwire under and looping itaround the ends of a conductors, retaining the loops of yielding tension,

' 6. That improvement in making electrical laid resistance element and with attached to the ends of the element gauges having cover strips with an interlead-conductors which comwinding, to form the resistance element. a strand of wire over a cover strip and looping it around the means on seidends of the lead-conductors. retaining the loops of wire under, tension, connecting the other ends of the lead-conductors to the electrical circuit for a. resistance measuring instrument, varying, by adjustment of at leiist one of the conductors, the length of the wire in the element between the conductors to the resistance value desired and relatively to the measured value, and joining together the loops of the wire and the conductors while they are held under tension.

"I. That improvement in making electrical strain gauges having cover strips with an interleid resistance element and with lend-conductors attached to the ends prises, adjustably supporting a, pair of lead-conof the element which comthe resistance value desired and energies ductors each having one of its ends provided with means. through which wire can be looped.

to form the resistance element, a. strand of wire over a cover strip and looping it around the means on said ends of the lead-conductors, retaining the loops 0! wire under tension, connecting the other ends of the lead-conductors to the electrical circuit for a resistance measuring instrument, varying, by adjustment of at least one of the conductors, the length of the wire in the element between'the conductors to relatively to the measured value, joining together the loops on the wire and the conductors while they are held un der tension, and severing the wire from the joints while it remains under tension.

' BERNARD J. CAMPBELL.

MA HILTON. 

